Choosing the Right Stone

As building material, stone has several positive features:
• It is easily available
• When correctly processed, stone is a stable, almost eternal material.
• It is non-flammable.
• Its appearance is natural and unique.

To enhance these features, the aesthetical and mechanical properties that determine the possible uses of a rock type must be understood in the planning phase. Only then can a long-lasting, beautiful result be achieved. There are couple of things to look at:

Outer Appearance
The outer appearance of rock types varies. Stone is a natural product. The texture and colouring of the material vary, contributing to its unique appearance. Surface treatments can either enhance or fade the surface colours and patterns.

Mechanical Properties
During dimensioning and attachment methods management, the mechanical properties of stone slabs must be taken into consideration. The weather resistance and susceptibility to discolouring are also estimated. The European Committee for Standardisation, CEN, has defined standard methods for the testing of mechanical properties. These are some descriptions for types of Mechanical Properties:

Water absorption
Water absorption is measured as mass percentage of dry weight. The water absorption of porous rocks is greater than that of dense rock types. As a result, porous rocks are susceptible to discolouring and have poor frost resistance. The values for dense rock types vary between 0.1 and 0.5 mass percents. The absorption value for porous rock types may exceed 20. A rock will have good frost resistance, when its water absorption is less than 0.5 mass percents.

Hardness
Measured by scratching the surface with mineral. Values are reported in Mohs' hardness, a scale of 1 to 10 Mohs, where the hardest rock will receive the value 10. This value can be used in estimating durability and persistence.

Thermal transformations
Thermal transformations are changes in volume that are caused by temperature changes. These properties are necessary in evaluating the need for expansion joints in structures. The thermal transformations of natural stone vary between 0.2 - 0.9 * 10^-5/C, and are less than with most building materials.

Humidity transformations
Humidity transformations are changes in volume that are caused by changes in humidity. Only the thin marble tiles have been shown to respond notably to changes in humidity. Marble tiles tend to bend unevenly when they get wet. For other rock types, this property bears little significance.

Abrasion resistance
Abrasion resistance must be considered when selecting the surface treatment, if the abrasion burden is significant. The European Union has issued a standard for abrasion resistance.

Breaking around a fixing point
The likelihood of stone breaking around a fixing point depends on several factors: the size of the stone slab, the quality and quantity of fixing points, and the hardness of the rock type. The sufficient value of 2 - 5 kN can usually be attained, when the slab is 30 mm thick.

Weather resistance
In the long run, weather resistance plays an important role in the durability and appearance of facades. Exposure to the climate and weather conditions may result in discolouring or colour changes, diminished polish, chemical and mechanical weathering, and in bent stone slabs. Different rocks have different weather resistance properties.

Igneous rocks (e.g. granite)
In igneous rocks, exposure to climate and weather conditions causes mainly surface discolouring. Surfaces with bush hammer finish are easily discoloured, but on a polished surface discolouring is close to non-existent.

Limestones and Marbles
Alkaline rock loses its shine and strength in acidic urban atmosphere. In only ten years, exposure to acid rains will result in severe weathering and cause limestone and marble to become brittle.

Choosing the Stone
Stone Type/Property
Granite - Marble - Calcite - Limestone density "a" - Limestone density "b" - Sandstone - Schists

Density min kN/m
25,6 - 25,9 - 17,6 - 25,6 - 22,4 - n/a

Water absorption max (mass percent)
0,4 - 0,75- 12 ,0 - 3,0 - 20,0 - 0,25

Compressive strength/Compression resistance min (MPa)
130,0 - 52,0 - 12,4 - 55,2 - 13,8 - n/a

Abrasion resistance min
10,0 - 10,0 - 10,0 - 8,0 - 8,0

Availability
It is advisable to determine the availability of a rock type before making the final selection. While planning, it is wise to have a few alternatives before confirming the order of the material. The project schedule should also be planned only after receiving confirmation on the availability of the chosen rock. Depending on the degree of processing, it may take 2 to 6 months before the ordered slabs are ready to be assembled.